RAID

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redundant array of independent disks. A method of mirroring or striping data on clusters of low-end disk drives; data is copied onto multiple drives for faster throughput; error correction; fault tolerance and improved mean time between failures. With the exception of RAID 0; all RAID levels provide automated recovery of data in the event of a disk failure. The RAID levels and their key features are: * RAID 0 - provides disk striping without parity information; data is written by segment across multiple disks sequentially until the end of the array is reached; and then writing starts at the beginning again. Provides greater logical disk capacity with faster access time on reads (multiple segments read simultaneously). However; RAID 0 provides no data redundancy if one drive fails; the entire disk array subsystem is unavailable. * RAID 1 - provides fault tolerance by using disk mirroring (also called shadowing). Each byte of data on a disk is duplicated on another physical drive; providing 100 percent data redundancy. RAID 1 provides immediate access to data when either the primary or secondary drive fails; but it has the highest cost of all RAID types; since duplicate hardware is required. * RAID 0+1 and 1+0 - combinations of RAID 0 and RAID 1 that provide the benefits of striping and fault tolerance (disk mirroring). RAID 0+1 (also known as RAID 01 or RAID 0/1) uses a mirrored configuration of two striped disk sets; RAID 1+0 (also known as RAID 10 or RAID 1/0) is a stripe across a number of mirrored sets. * RAID 2 - eliminates the 100 percent redundancy overhead of RAID 1 by using a powerful error detection and correction code (Hamming); with bits of the data pattern written across multiple disks. * RAID 3 - similar to RAID 2; but uses a single check disk per group that contains the bit parity of the data disks; data is interleaved across all disks. Because disk reads are performed across the entire array and all data is transferred to the controller in parallel; RAID 3 is well suited for applications that require high data read/write transfer rates for large sequential files. * RAID 4 - instead of interleaving blocks of data across all drives; writes the first block on drive 1; the second block on drive 2; and so on. This technique dramatically improves read time; since many reads are single block (single drive); freeing other drives for additional read requests. * RAID 5 - eliminates the dedicated parity drive by writing parity with the data across all drives in the array. Consequently; the single-write restriction and some performance degradation of RAID 1 through RAID 4 are eliminated. If a drive fails; the controller can rebuild the data from the parity and data on the remaining drives. * RAID 6 - provides two-disk parity and one spare; so that two simultaneous disk failures per array of disks can be tolerated. With the occurrence of a failure; a spare is brought online and transparent reconstruction begins automatically in the background with negligible impact on performance.